`ENABLE_VULKAN_DEBUGGER` is the new option to enable building of the Vulkan debugger, which defaults to off.

`cppdap` is a new library that implements the debugger protocol.

Added a stub .cpp file to try and detect any build systems that are recursively globbing into the new src/Vulkan/Debug directory.

Actual debugger implementation will come in future changes.

Bug: b/145351270
Change-Id: I48619f1d8352c0525c8adbaa4bc3f2373a237d61
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38890
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>